There are increasing tendencies in CATV (cable television) broadcasting systems or satellite television broadcasting systems, for instance, to use a digital transmission technique for transmitting image data in place of a conventional analog transmission technique. FIG. 1 shows a digital image data receiver in such a digital television transmission system using such a satellite. In FIG. 1 transmission signals from 11.7 GHz to 12.3 GHz, which are processed using a QPSK (quadrature phase shift keying) modulation, are transmitted from a satellite and then received by a satellite antenna 11. Groups of received signals are translated into 950 MHz to 1450 MHz by a frequency converter 12, an then amplified at an amplifier 13.
The received broadcasting signal output from the amplifier 13 is converted into an intermediate frequency signal in the 140 MHz band at a mixer 15 for mixing the received signal with a local oscillation signal from a variable local oscillation VCO (voltage controlled oscillator) 16, after being suppressed its image frequency regions at a pre-selector 14. The output from the mixer 15 is amplified at a variable gain amplifier 17 and then limited in a desirable band width at a BPF (band pass filter 18). The intermediate frequency signal is branched off on two paths at a separator 19. One of the intermediate frequency signals on the paths output from the separator 19 is supplied to a phase detector 20, and the other of it is supplied to another phase detector 21.
The phase detector 20 orthogonally detects an I-axis base-band signal by a phase detection on the input intermediate frequency signal based on an oscillation signal supplied from the VCO 22 through a separator 28. Also, the other phase detector 21 orthogonally detects a Q-axis base-band by the phase detection on the other intermediate frequency signal based on a 90.degree. phase shift signal of the oscillation signal from the separator 28 through a 90.degree. phase shifter 24.
Further, the I axis and Q axis base-band signals output from the phase detectors 20, 21 are each shaped their waveforms at LPFs (low pass filter) 25, 26, and then digitized at A/D (analog to digital) converters 27, 28. The digitized signals are supplied to a digital processor 29 which demodulates them into their corresponding data sequences together with a clock reproduction and a carrier reproduction. Thus, the demodulated data sequences are output through output terminals 30, 31. Further, the digital processor 29 generates a frequency control voltage signal for controlling the oscillation frequency of the VCO 22 which is used for the phase detection processings in the phase detectors 20, 21. The digital processor 29 also generates a control signal for controlling the gain of the amplifier 17.
Here, the band width of a satellite transponder is about 24 MHz to 40 MHz, and, in the analog video transmission, one transponder is typically occupied for the transmission of one video channel with an FM modulation. However, if a digital image compression technique is employed, it is possible to multiplex up to eight channels since one video channel is able to be transmitted by about 4 Mbps to 10 Mbps.
As such a multiplex system, there are an MCPC (Multi Channel Per Carrier) system which transmits a plurality of digital image data by multiplexing them on one carrier and an SCPC (Single Channel Per Carrier) system which transmits one channel image data by dividing it into a plurality of frequency bands. It is expected that these two multiplex systems will be used together in taking account of their features. And also it is expected that the analog transmission system will be used in the same satellite together with the multiplex systems.
The conventional digital image data receiver, as shown in FIG. 1, performs a channel tuning by varying the oscillation frequency of the variable frequency local oscillation VCO 16 in adjusting to the carrier central frequency of the received signal. In the tuning of the digital image data multiplexed in the MCPC system, there rises no problem since the central frequency of the tuned signal band, as indicated by a solid line in FIG. 2(a), agrees with the central frequency fc of the satellite transponder frequency band, as indicated by a broken line in the same FIG. 2(a).
However, in the SCPC system there are a plurality of data carriers D1 to D3, as shown by solid lines in FIG. 2(b) within the one transponder band, as shown by the broken line in the same FIG. 2(b) (three carriers are shown in the drawing's case). To select a desirable carrier from these data carriers D1 to D3, the oscillation frequency of the local oscillation variable VCO 16 must be offset so as to make the central frequency of the intermediate frequency signal output from the mixer 15 agree with the central frequency fd1, fd2 or fd3 of the desired carrier.
Accordingly when the left side data carrier D1 is selected, as shown in FIG. 2(c), a possibility of signal S1 of an adjacent channel entering within the intermediate frequency band of the data carrier D1 becomes higher. Further, it the SCPC system it employs a backoff for limiting the total power of the transponder lower than a prescribed power level to account for signal distortions. As a result, when the adjacent channel is the type of the MCPC system or the analog transmission system there also arises a level difference.
Accordingly, there is required a narrow band filter in the reception of signals in the SCPC system. However, it is technically difficult to provide such a 1 GHz band pre-selector 14 with a sufficiently narrow band characteristics. Thus, the mixer 15 is subject to the crosstalk of adjacent channel signals. To avoid such a crosstalk of the adjacent channel signals, a sophisticated intermodulation distortion must be performed based on the level difference between the desired and undesired signals. Although it is possible to make the band width of the BPF 18 narrower, in the reception of signals shared with signal in the MCPC system or signals with different data rates, it is required to change over many frequency bands, resulting in a sophisticated receiver circuit arrangement having increased size.
As described above, in the conventional digital data receiver, it is difficult to satisfactorily receive the whole digital data transmitted by mutually different multiplex systems such as a time division multiplex system or a frequency division multiplex system, at a simple construction.